Air pressure of a tire is one of the elements that allow a vehicle to safely travel. When the air pressure of the tire is low, a vehicle easily slides, and as a result, there is a possibility that the life-span of the tire is shortened and ride comfort and braking force significantly deteriorate. When the air pressure of the tire decreases, functional problems including deterioration of fuel efficiency, tire wear, and the like may occur. In addition, when the decrease in air pressure is significant, there is a possibility that vehicle damage and danger to human life such as an accident occurrence caused by a driving inoperability state or tire rupture will occur.
However, since most drivers cannot recognize a change in air pressure of the tire, a tire pressure monitoring system (TPMS) which is a tire pressure monitoring system announcing the change in pressure of the tire to the drivers in real time has been developed.
In recent years, the tire pressure monitoring system (TPMS) is mounted on a vehicle, which detects the decrease in air pressure of the tire mounted on the vehicle and announces the detected decrease in air pressure to the driver.
The tire pressure monitoring system (TPMS) announces the decrease in pressure of the tire to the driver to allow the driver to check a pressure state of the tire, thereby solving the problem.
The TPMS may be generally classified into a direct scheme and an indirect scheme.
The direct scheme of TPMS installs a pressure sensor in a tire wheel to directly measure the air pressure of the tire. The direct scheme of TPMS announces the change in air pressure of the tire, which is measured from the pressure sensor attached to the tire to the driver. The indirect scheme of TPMS indirectly estimates the change in air pressure of the tire through a change of a response characteristic (for example, a rotational velocity or a frequency characteristic of the rotational speed) of the tire, which is generated when the air pressure decreases and announces the indirectly estimated change to the driver. Even though the direct scheme of TPMS may accurately detect the lowering of the air pressure of the tire, a dedicated wheel is required and there may be a problem in the performance in an actual environment. Therefore, the direct scheme of TPMS has technical and costly problems.
The indirect scheme of TPMS is a method that estimates the air pressure of the tire from rotation information of the tire. The indirect scheme of TPMS may be, in detail, classified into a dynamic loaded radius (DLR) analysis scheme and a resonance frequency method (RFM) analysis scheme again. They may be briefly called a radius analysis scheme and a frequency analysis scheme.
In a frequency analysis scheme, when the air pressure of the tire decreases, a difference from a tire having a normal air pressure is detected by using when a frequency characteristic of a rotational velocity signal of a wheel is changed. In the frequency analysis scheme, based on a resonance frequency which may be acquired by frequency analysis of the rotational velocity signal of the wheel, when the relevant resonance frequency is calculated to be lower than a reference frequency estimated while initialization, it is determined that the air pressure of the tire decreases.
In a radius analysis scheme, by using a phenomenon in which a dynamic loaded radius of the depressurized tire decreases while driving, and as a result, the tire rotates more rapidly than the normal tire, the pressure decrease is detected by comparing rotational velocities of four tires. In the radius analysis scheme of the tire pressure monitoring system, since it is determined whether the tire is depressurized based on a wheel velocity, the wheel velocity exerts a largest influence on the determination of the depressurization.
The direct scheme of TPMS may accurately sense the air pressure of the tire, but the life-span of a battery is limited and whenever the tire is replaced, the direct scheme of TPMS needs to be installed again. In the direct scheme of TPMS, since a pressure sensor is attached, imbalance of the tire may occur and problems including radio frequency interference and the like may occur. Further, since the direct scheme of TPMS is a scheme that mounts the sensor on the tire to measure the air pressure, the indirect scheme of TPMS has an advantage in that the direct scheme of TPMS measure accurate pressure. On the contrary, the direct scheme of TPMS is constituted by various components including a pressure measurement sensor mounted on the tire, a wireless communication unit for transmitting a measurement value in a general wireless scheme, and the like. Therefore, the direct scheme of TPMS is more expensive and further, higher in failure rate than the indirect scheme of TPMS.
Meanwhile, the indirect scheme of TPMS is a scheme that estimates a loss in air pressure by using a wheel speed sensor which is mounted on the vehicle to measure a wheel speed. In the indirect scheme of TPMS, since the TPMS may be implemented only by an algorithm, additional hardware is not required, which results in a reduced cost. Further, just a little maintenance cost is consumed. The indirect scheme of TPMS has better price competitiveness than the indirect scheme of TPMS. However, since the resonance frequency of the indirect scheme of TPMS varies depending on the wheel speed, accuracy of the indirect scheme of TPMS slightly deteriorates. Since the estimated change in air pressure of the tire may be different from an actual change, the indirect scheme of TPMS may send a false alarm to the driver.
Meanwhile, when a frequency logic is calculated, the indirect scheme of TPMS may accumulate frequencies for every speed section and obtain a mean thereof, to increase the performance. The frequencies accumulated as described above may increase or decrease in proportion to the speed. When the frequencies monotonically increases or monotonically decreases, a base line of the frequency may be clearly evaluated.
However, in some cases, the frequencies accumulated as described above may not increase or decrease in proportion to the speed. In this case, the base line of the frequency may vary or is not accurate, so that accurate frequency analysis may not be performed.
Therefore, in order to set a base line for accurately analyzing the frequency, a logic which may correct the frequency accumulated for frequency sections to increase or decrease in proportion to the speed is required.